The present invention relates to a variable venturi-type carburetor.
A variable venturi-type carburetor comprises a suction piston, projecting into the intake passage of the carburetor, and a compression spring biasing the suction piston toward the intake passage. The suction piston moves so that the difference between the pressure in the intake passage located upstream of the suction piston and the vacuum in the intake passage located downstream of the suction piston becomes equal to a fixed value determined by the spring force of the compression spring. In an engine equipped with such a carburetor, in order to obtain a good combustion when a lean air-fuel mixture is used, it is necessary to promote the atomization of fuel by increasing the velocity of air passing through the tip face of the suction piston. In this regard, if the spring force of the compression spring is strengthened, it is possible to increase the velocity of air passing through the tip face of the suction piston. However, if the spring force of the compression spring is strengthened, when the amount of air fed into the cylinder of the engine is small, as at the time of idling, the flow area of the intake passage becomes excessively small and, thus, the flow resistance of the intake passage becomes large. Thus makes it impossible to feed air into the cylinder of the engine in the amount necessary for the engine idling operation, and thus, makes idling operation impossible.